Conventional contact adhesives of the prior art, for example based on polychloroprene, styrene-butadiene-styrene block copolymers, styrene-acrylate copolymers or polyurethanes, are predominantly solvent-containing adhesives.
For ecological and economic reasons, as well as for reasons of occupational safety and hygiene, there is a growing need for suitable aqueous dispersions which can be processed to corresponding solvent-free adhesive formulations.
State of the art are solvent-free contact adhesive formulations based on polychloroprene as the main constituent of the adhesive composition. An important quality feature for aqueous contact adhesive formulations is good wet-on-wet bondability on a wide variety of substrates (high adhesiveness or reactivity of the adhesive directly after application of the adhesive) combined with a high initial strength directly after joining of the substrates to be bonded.
The expression high initial strength is closely linked to the definition of a contact adhesive. Contact adhesives can in principle be applied at ambient temperature to substrates at ambient temperature. A sufficiently high initial strength is to be understood as being a strength directly after joining of the substrates that is sufficiently high for further processing of the joined parts so that slipping of the joined parts in the subsequent processing operation is avoided, or the bonded substrates exhibit a strength below the adhesion and cohesion strength of the adhesive layer so that, when a load is applied to the joined object, the joined part is generally destroyed before the adhesive layer.
Typically, commercially available dispersions must be destabilised or activated in order to achieve an adequate initial strength. A main problem in the formulation of reactive adhesives is the difficulty of linking adequate reactivity with adequate storage, temperature, shear and transport stability.
Conventional anionically stabilised dispersions or formulations based on polychloroprene usually have their pH value reduced in a targeted manner for that purpose and are consequently destabilised or activated and only then rendered capable of wet-on-wet adhesion. A disadvantage is that, as the pH value falls, the storage, temperature, shear and transport stability of such formulations also falls. Furthermore, oxidative and hydrolytic processes in the dispersions continuously set in motion further destabilising processes, which undoubtedly cause coagulation, or greatly reduced storage, temperature, shear and transport stability.
For the purposes of the present invention, activation is understood as meaning the establishment of initial adhesiveness in adhesive formulations, as well as the initiation of the binding or crosslinking process in the production of hydrogels, foams, coatings and seals.
It is also state of the art that anionically stabilised polymer dispersions are not permanently stable because the resinates used as the main emulsifiers and/or the fatty acids used as emulsifiers, as a result of conversion into their acid form, are no longer available as emulsifiers and effective stabilisation of the latex is no longer ensured.
It is further generally known that the addition of relatively large amounts of pH-stable, anionic emulsifiers based, for example, on sulfate or sulfonate, as well as non-ionic emulsifiers, in principle supports pH value stability at a pH value <9, but at the expense of the contact adhesiveness of the formulations due to increased foam formation and suppression of the coagulation of the latex particles or impairment of the film-forming properties.
A further method for destabilising anionically stabilised polychloroprene-containing dispersions in a targeted manner consists in adding additional electrolytes, which on the one hand increase the ionic strength of the water, and thereby reduce the effectiveness of the anionic emulsifiers, and render the emulsifiers that are present ineffective in their function as emulsion stabilisers in a targeted manner by coordinative and complexing effects and/or precipitation. For example, there are used for this purpose monovalent, divalent or trivalent inorganic salts, for example KCl, NaCl, Na2SO4, ZnCl2, MgCl2, CaCl2, AlCl3, Al2(SO4)3, or inorganic or organic acids. A disadvantage which can be mentioned here too is reduced storage, temperature, shear and transport stability. Such formulations are referred to hereinbelow as 1-component adhesive formulations.
Of the acids and salts mentioned in the prior art which are suitable for the targeted destabilisation or activation of polychloroprene latices, only amino acids such as glycine or boric acid can be added directly without diluting the concentration of the desired latex. All other substances (salts, acids, buffers) must be added diluted to a greater or lesser extent in water in order to avoid direct coagulation of the latex on addition of the components. However, the use of glycine or boric acid likewise has disadvantages because, for example, glycine when used in high concentrations to achieve a low pH value has a strongly adverse effect on the bonding properties of the formulation owing to its surface-active action, and boric acid is in future to be classified as toxic. Post-activation of highly viscous formulations is possible by means of the mentioned acids typically only with great difficulty owing to their only slow distribution in the highly viscous formulations and the resulting excessive concentrations which, in combination with the mechanical stress in stirring apparatuses, leads to coagulate formation.
Overall, adhesive formulations based on anionically stabilised polychloroprene dispersions exhibit reduced storage, temperature, shear and transport stability even at a pH value <10.
A further problem in the formulation of 1-component adhesives based on anionically stabilised commercially available polychloroprene dispersions is that they liberate HCl in significant amounts during storage. This additional stress by additional lowering of the pH value leads in the case of ready-formulated reactive 1-component adhesive formulations to a further limitation of the storage, temperature, shear and transport stability.
Accordingly, in addition to the stability of the formulation, the pH value stability of a finished applied adhesive is also of interest, because HCl liberation can cause possible discolouration in the adhesive and on pH-sensitive substrates. A possible acid attack is conventionally counteracted by adding divalent, predispersed metal oxides (ZnO, MgO, CaO), in some cases present as nanoparticles, and/or aminic acid acceptors (e.g. described in WO-A 2004/106422).
Furthermore, application US-A 2003/221778 describes the use of silicic acid/water glass suspensions for improving the wet adhesiveness and the initial strength and final strength of polychloroprene contact adhesive formulations. In addition, viscosities are established in a targeted manner via silicic acid suspensions with the aid of pH value and mono- and/or di-valent ions. The silicic acids disclosed in US-A 2003/221778 are sensitive both to a low pH value (<9) and to high concentrations, in particular of divalent ions/metal oxides (ZnO, MgO).
The use of carbon dioxide as a neutralising and coagulating agent in anionically stabilised dispersions has already been disclosed many times in the prior art. There, carbon dioxide is used as a weak acid for neutralising an anionically stabilised polymer dispersion, with the aim of direct coagulation in order to obtain a solid from the dispersion or for the purpose of creaming by agglomeration and freezing out of dispersions in order to improve the properties of the product for the production of latex foams. Fundamental for the described uses of carbon dioxide was always that destabilisation of an anionic dispersion by carbon dioxide has the result of effecting partial or final coagulation after a very short time. Alternatively, in the case of the creaming aid, the latex was subsequently rendered basic again, partly in order to improve the durability.
The problem of storage, temperature, shear and transport instability caused by targeted destabilisation/activation of an anionically stabilised adhesive formulation (1-component formulation) can be avoided if the destabilisation/activation does not take place until immediately before, with or by the application (2-component formulation or process). This activation is then typically effective immediately and leads to the direct coagulation or breaking of the formulation during application or on the substrates. As a result, substrates wetted therewith can immediately be joined under pressure with outstanding wet-on-wet adhesive properties.
These 2-component adhesives are conventionally composed of an anionically stabilised dispersion or formulation containing polychloroprene and a separate destabilising or activating component.
An example of this process is the “spray-mix” process. In this process, the adhesive and a coagulating agent are fed separately in a spray gun, mixed in the spray jet and coagulated while still in flight and/or on the joined part. As destabilising component there can be used, for example, concentrated aqueous solutions of alkali metal, alkaline earth metal ions, trivalent aluminium ions, inorganic/organic acids, or mixtures thereof.
Disadvantages of this process are the high outlay in terms of apparatus, the mixing process, which is susceptible to errors, and the typically high ion concentrations in the resulting adhesives, which can lead to increased equilibrium water swelling or—in the case of the use of acids—to corrosion in connection with metal substrates when low pH values are established. Furthermore, as a result of this process, increased amounts of water are typically carried into the formulations by the second component, which can lead to problems with drying processes and excessive residual moisture contents in the products, for example with the risk of corrosion of sensitive substrates. The process is comparatively time- and cost-intensive and is therefore unsatisfactory from an economic point of view.